Which valve prevents blood from flowing back into the left ventricle after contraction?

• A. Tricuspid valve
• B. Mitral valve
• C. Aortic semilunar valve
• D. Pulmonary semilunar valve

Answer: (C)

Unidirectional flow through the heart is maintained by valves that close after the heart muscle contracts. After the left ventricle contracts to push blood into the aorta, the aortic semilunar valve closes to prevent blood from flowing back into the ventricle. This contrasts with the mitral valve, which lies between the left atrium and ventricle and stops backward flow into the atrium during systole, the tricuspid valve between the right atrium and ventricle, and the pulmonary semilunar valve between the right ventricle and pulmonary artery.